Exhaust gas after-treatment systems and methods aim to reduce exhaust emissions, such as carbon monoxide (CO), unburned hydrocarbons (UHC), NOx and particulate emissions. After-treatment systems may include three-way catalysts (TWC), oxidation catalysts, particulate filters, lean NOx traps, and catalytic converters, such as a selective catalytic reduction catalyst (SCR catalyst) or a urea-selective catalytic reduction catalyst, located downstream of an internal combustion engine.
Nitrogen oxides (NOx) emissions may include nitric oxide (NO) and nitrogen dioxide (NO2). These gases may be harmful to the environment and are restricted according to emission standards, such as, Tier 2, EURO V, Euro VI, low emissions vehicle (LEV) I, LEV II and LEV III. Existing NOx reduction technologies, such as, lean NOx traps and catalytic converters, may reduce NOx emissions to cleaner substances, such as, nitrogen (N2) and water (H2O).
Exhaust gas may have high temperatures, for example, in a range of from approximately 200° C. to approximately 400° C. for a warmed up diesel engine. After-treatment systems may have optimal performance (e.g., a maximum reduction in undesirable emissions) at high temperatures in ranges of from, for example, 250° C. to 350° C. for catalytic converters, 600° C. to about 700° C. for diesel particulate filters (sufficiently hot to burn soot), and 250° C. to about 500° C. for lean NOx traps (sufficiently hot to desulfate or remove sulfur from the trap). These temperature ranges for optimal after-treatment performance may vary depending upon the type of engine and after-treatment process.
These elevated temperatures are typically achieved through the use of an oxidation catalyst or in the case of a diesel engine a diesel oxidation catalyst (DOC), which generates an exothermal reaction with raw hydrocarbons that are included in or injected into the exhaust stream, such as by in-cylinder injection or external injection directly into the exhaust stream.
If the temperatures of the after-treatment systems fall outside these ranges, the after-treatment performance may be compromised and undesirable emissions may increase. For example, each time a car is started, idle, or stops and then starts, the after-treatments system may cool and may be unable to optimally purify the exhaust, thereby contributing to elevated pollutant levels in the air.